Rett Syndrome (RTT) is an Autism Spectrum Disorder (ASD) of known monogenic cause. It represents one of the leading causes of intellectual and developmental disabilities in females. RTT is caused by mutations or duplications of the gene encoding MeCP2, a methyl DNA binding protein thought to modulate global gene expression. Mice with loss of function of MeCP2 (knock-out) have a range of physiological and neurological abnormalities that mimic the human syndrome. Despite the fact that most RTT-causing mutations are clustered at the methyl-CpG binding domain of MeCP2, it is not known if disruption of methyl-CpG recognition alone is sufficient to cause RTT. Moreover, the mechanisms by which MeCP2 dysfunction leads to RTT remain poorly understood. Recently, we have developed a knock-in mouse carrying a missense mutation of MeCP2 at Threonine 158 (T158A). T158 is located at the methyl-CpG binding domain of MeCP2 and is the most common missense mutation associated with human patients. Preliminary studies suggest T158A mutation impairs the ability of MeCP2 to recognize methylated DNA. In addition, T158A knock-in mice appear to show similar symptoms to MeCP2 knock-out mice. Thus, within the scope of this New Program Development Award, we plan to focus on the following specific aims: 1) Phenotypic characterication of the MeCP2 T158A knock-in mice in comparison to MeCP2 knock-out mice. 2) Dissection of the molecular and cellular functions of MeCP2 with MeCP2 T158A knock-in mice. In the long run, we hope to not only gain insights into the pathogenic mechanisms of RTT, but also uncover therapies and cures for ASD.